![Trickle vents from the outside]()
Whole Dwelling
Apart from on-demand purge and extraction, there should be a minimum flow of ventilation for rooms, dependent on floor area and how many stories in the building.
Whole dwelling ventilation should distribute fresh air throughout the house without opening a window to disperse moisture and pollutants such as VOCs and carbon dioxide.
The distribution of air can be provided by a continuous supply fan or background ventilators depending on how airtight the property is:
- Natural background ventilators for less airtight houses, including trickle vents for windows, air bricks and passive stack ventilation.
- Continuous supply for airtight houses includes Mechanical Ventilation with Heat Recovery (MVHR).
As a note, internal doors are required to have space underneath for airflow through the home.
| Whole house ventilation rates by number of bedrooms |
|---|
| Number of bedrooms | Minimum ventilation rate (Litres per second) |
|---|
| One | 19 l/s |
| Two | 25 l/s |
| Three | 31 l/s |
| Four | 37 l/s |
| Five | 43 l/s |
Note that the whole dwelling ventilation rates have increased considerably since the previous version.
Requirements for Trickle Vents in Windows
To explain what size trickle vents should be for a window or a room, you first have to understand that trickle vents are not measured in physical size. They are measured in the industry standard of equivalent area.
Equivalent area means the amount of air that can pass through the vent.
“Equivalent area is a measure of the aerodynamic performance of a ventilator. It is the area of a sharp edged circular orifice through which air would pass at the same volume flow rate, under an identical applied pressure difference, as through the opening under consideration.” Building Regulations Part F.
| Minimum size of background ventilators (trickle vents) in Equivalent Area (EA) |
|---|
| Multiple floors
Minimum | Single floor
Minimum |
|---|
| Living spaces | 8,000mm2 | 10,000mm2 |
| Kitchen | 8,000mm2 | 10,000mm2 |
| Bathroom | 4,000mm2 | 4,000mm2 |
| Utility room & toilet | No minimum | No minimum |
The number of ventilators installed in living spaces and kitchens should be no less than five (four in one-bedroom houses). For houses with continuous mechanical extract ventilation, the number of trickle vents must be the number of bedrooms plus two (E.g. Three for a one-bedroom house and five for a four-bedroom).
The table above also applies to ‘habitable rooms’ which is defined as ‘a room used for dwelling purposes'. Hallways and landings do not fall into this category.
There are exceptions to the table above for houses that only have one exposed elevation (e.g. back-to-back terraced houses). For houses with 70% of the windows on one side of the house (e.g. end back-to-back terraced houses), for kitchens with no external window or facade (e.g. in a basement), or for open plan kitchens/living rooms. Please refer to the full Approved Document Part F.
Trickle vents should be positioned 1.7m above the floor to help prevent draughts.
How many trickle vents do I need? – examples:
- A three-bedroom house without trickle vents
Habitable rooms must each have five background vents with a combined EA of 8,000mm2. - A three-bedroom house with existing trickle vents
Habitable rooms must each have background vents at the same size as the previous windows. - A two-bedroom house with a mechanical extract system
Habitable rooms must each have four background vents with a combined EA of 4,000mm2. - A one-bedroom bungalow without trickle vents
Habitable rooms must each have four background vents with a combined EA of 10,000mm2.
Less Airtight Houses
Houses that are considered ‘less airtight’ means they have:
- A design air permeability higher than 5m3/(h·m2) at 50Pa.
- An as-built air permeability higher than 3m3 /(h·m2) at 50Pa.
In houses that are defined above, they have the option of natural background ventilation.
| Types of ventilation |
|---|
| Houses covered |
|---|
| Natural ventilation | Less airtight houses |
| Continuous mechanical extract ventilation | All houses |
| Mechanical ventilation with heat recovery | All houses |
![Purge ventilation open window]()
Purge
Purge ventilation is the ability to open a window or door that can allow enough fresh air directly from the outside. The minimum rate for purge ventilation is four air changes per hour in an internal space.
Purge ventilation is required to quickly remove odours and fumes from cooking, cleaning or DIY jobs. Purge is also used to remove excess heat from a room to quickly cool the room down.
All habitable rooms must provide the ability to purge and where an outside window is not installed, a mechanical extract ventilation system can be used. Windows must have a minimum opening size.
| Purge ventilation openings |
|---|
| Minimum total area of opening |
|---|
| Hinged or pivot window opening between 15-30 degrees | 1/10 of room floor area |
Hinged or pivot window opening more than 30 degrees
Opening sash windows
External doors | 1/20 of room floor area |
Other Considerations
The ventilation must also achieve the following where reasonably practicable:
- Low noise
- Easy maintenance access
- Protection from cold draught
It should be noted that Part F also interacts with other Building Regulations and they may supersede in some instances:
- Part L Energy Efficiency and the control of infiltration must be factored in when choosing a ventilation system
- Part O Excess Heat should supersede Part F where the standards for purge ventilation in Part O are higher.